WO2017048088A1 - Channel estimation method in dual mobility environment, and user equipment - Google Patents

Abstract

A disclosed feature of the present specification provides a method for user equipment estimating a channel. The method may comprise the steps of: receiving two or more cell-specific reference signals (CRSs); dividing the two or more CRSs into two groups depending on an antenna port number; executing Doppler frequency tracking with respect to the respective divided groups; and estimating a single frequency network (SFN) channel based on the result of Doppler frequency tracking executed for the respective groups.

Description

Channel Estimation Method and User Device in Dual Mobility Environment

The present invention relates to mobile communications.

The 3rd Generation Partnership Project (3GPP) long term evolution (LTE), an improvement of the Universal Mobile Telecommunications System (UMTS), is introduced as a 3GPP release 8. 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in downlink and single carrier-frequency division multiple access (SC-FDMA) in uplink. A multiple input multiple output (MIMO) with up to four antennas is employed.

As disclosed in 3GPP TS 36.211 V10.4.0 (2011-12) "Evolved Universal Radio Access (E-UTRA); Physical Channels and Modulation (Release 10)", in LTE, a physical channel is a downlink channel PDSCH (Physical Downlink Shared) Channel (PDCCH), Physical Downlink Control Channel (PDCCH), Physical Hybrid-ARQ Indicator Channel (PHICH), Physical Uplink Shared Channel (PUSCH) and PUCCH (Physical Uplink Control Channel).

On the other hand, in recent years, the introduction rate of high-speed railway is rapidly increasing worldwide. As a result, demand for high-speed data communication is also increasing in an environment moving at high speed. In the following description, the case where the mobile terminal having mobility is located in the moving means will be described as a dual mobility environment.

However, the current LTE or LTE-Advance (LTE-A) standard lacks a definition for an environment that moves at high speed. In particular, according to the current LTE or LTE-A, estimation of a single frequency network (SFN) channel in a high-speed mobile environment is a major factor of performance degradation. Therefore, there is a need for a solution capable of effectively estimating the SFN channel in a dual mobility environment.

Accordingly, one disclosure of the present disclosure aims to provide a method for effectively performing channel estimation in a dual mobility environment.

Another object of the present disclosure is to provide a user device capable of performing channel estimation effectively in a dual mobility environment.

In order to achieve the above object, one disclosure of the present specification provides a method for estimating a channel by a user equipment. The method includes receiving two or more Cell-specific Reference Signals (CRSs); Dividing the two or more CRSs into two groups based on an antenna port number; Performing doppler frequency tracking on each of the divided groups; And estimating a single frequency network (SFN) channel based on the Doppler frequency tracking result performed for each group.

In the dividing into two groups, the two or more CRSs may be divided into an odd group and an even group, wherein the CRS having an odd antenna port number may be divided into an odd group, and the CRS having an even antenna port number may be divided into an even group. have. In particular, the CRSs divided into odd groups and even groups may be received through different remote radio heads (RRHs).

The dividing into two groups may be performed by dividing the two or more CRSs into two groups only when an indicator for notifying communication using the SFN channel is received. In this case, the indicator may be extracted from a master information block (MIB) received through a physical broadcast channel (PBCH). In particular, the indicator can be transmitted on one bit replaced to transmit the indicator among the spare bits in the MIB.

The method may further include performing automatic frequency control based on a weighted-averaged value of the SFN channel estimation result.

In order to achieve the above object, another disclosure of the present disclosure provides a user equipment (User Equipment) for estimating the channel. The user device includes a radio frequency (RF) unit for transmitting and receiving a radio signal; And a processor controlling the RF unit. The processor controls the RF unit to receive two or more Cell-specific Reference Signals (CRSs); Divide the two or more CRSs into two groups based on the antenna port number; Perform Doppler frequency tracking for each of the separated groups; And estimating a single frequency network (SFN) channel based on the Doppler frequency tracking result performed for each group.

According to the disclosure of the present specification, effective dual frequency tracking may be performed in a dual mobility environment to prevent performance degradation of SFN channel estimation.

1 is an exemplary view showing a wireless communication system.

2 shows a structure of a radio frame according to FDD in 3GPP LTE.

3 is an exemplary diagram illustrating a resource grid for one uplink or downlink slot in 3GPP LTE.

4 shows a structure of a downlink subframe in 3GPP LTE.

5 shows a structure of an uplink subframe in 3GPP LTE.

6 through 8 illustrate some examples of a cell-specific reference signal (CRS) structure when using one or more antennas.

9 and 10 show some examples of Dedicated RS (DRS) structures.

11 shows an example of a DeModulation RS (DMRS) structure.

12 illustrates an example of a dual mobility environment.

13A to 13C show some simulation results showing channel characteristics of two Doppler frequencies.

14A and 14B are some simulation results showing the performance of channel estimation in a dual mobility environment.

15 conceptually illustrates a channel estimation method according to the present specification.

16 shows an example of a structure of a conventional CRS channel estimator.

17 shows an example of a structure of a CRS channel estimator according to the present specification.

18 is a flowchart illustrating a channel estimation method according to the present specification.

19 is a block diagram illustrating a wireless communication system in which the disclosures herein are implemented.

It is to be noted that the technical terms used herein are merely used to describe particular embodiments, and are not intended to limit the present invention. In addition, the technical terms used in the present specification should be interpreted as meanings generally understood by those skilled in the art unless they are specifically defined in this specification, and are overly inclusive. It should not be interpreted in the sense of or in the sense of being excessively reduced. In addition, when the technical terms used herein are incorrect technical terms that do not accurately represent the spirit of the present invention, it should be replaced with technical terms that can be understood correctly by those skilled in the art. In addition, the general terms used in the present invention should be interpreted as defined in the dictionary or according to the context before and after, and should not be interpreted in an excessively reduced sense.

Also, the singular forms used herein include the plural forms unless the context clearly indicates otherwise. In the present application, terms such as “consisting of” or “comprising” should not be construed as necessarily including all of the various components, or various steps described in the specification, wherein some of the components or some of the steps It should be construed that it may not be included or may further include additional components or steps.

In addition, terms including ordinal numbers, such as first and second, as used herein may be used to describe various components, but the components should not be limited by the terms. The terms are used only for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, the first component may be referred to as the second component, and similarly, the second component may also be referred to as the first component.

When a component is said to be "connected" or "connected" to another component, it may be directly connected or connected to that other component, but there may be other components in between. On the other hand, when a component is said to be "directly connected" or "directly connected" to another component, it should be understood that there is no other component in between.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings, and the same or similar components will be given the same reference numerals regardless of the reference numerals, and redundant description thereof will be omitted. In addition, in describing the present invention, when it is determined that the detailed description of the related known technology may obscure the gist of the present invention, the detailed description thereof will be omitted. In addition, it should be noted that the accompanying drawings are only for easily understanding the spirit of the present invention and should not be construed as limiting the spirit of the present invention by the accompanying drawings. The spirit of the invention should be construed to extend to all changes, equivalents, and substitutes in addition to the accompanying drawings.

Hereinafter, the present invention is applied based on 3rd Generation Partnership Project (3GPP) 3GPP Long Term Evolution (LTE) or 3GPP LTE-A (LTE-A). This is merely an example, and the present invention can be applied to various wireless communication systems. Hereinafter, LTE includes LTE and / or LTE-A.

The user equipment used below may be fixed or mobile, and may include a terminal, a user equipment (UE), a wireless device, a mobile terminal (MT), a mobile equipment (ME), and an MS. (mobile station), user terminal (UT), subscriber station (SS), handheld device (Handheld Device), may be called in other terms, such as AT (Access Terminal).

In addition, the term base station (hereinafter referred to as base station) generally refers to a fixed station (fixed station) for communicating with user equipment, and includes an evolved-NodeB (eNB), a base transceiver system (BTS), and an access point Etc. may be called.

1 is an exemplary view showing a wireless communication system.

As can be seen with reference to FIG. 1, a wireless communication system includes at least one base station 10. Each base station 10 provides a communication service for a particular geographic area (generally called a cell) 10a, 10b, 10c.

The user device 20 typically belongs to one cell, and the cell to which the user device 20 belongs is called a serving cell. A base station that provides a communication service for a serving cell is called a serving BS. Since the wireless communication system is a cellular system, there are other cells adjacent to the serving cell. Another cell adjacent to the serving cell is called a neighbor cell. A base station that provides communication service for a neighbor cell is called a neighbor BS. The serving cell and the neighbor cell are relatively determined based on the wireless device.

Hereinafter, downlink means communication from the base station 10 to the user device 20, and uplink means communication from the user device 20 to the base station 10. In downlink, the transmitter may be part of the base station 10 and the receiver may be part of the user device 20. In uplink, the transmitter may be part of the user device 20 and the receiver may be part of the base station 10.

Hereinafter, the LTE system will be described in more detail.

2 is 3GPP In LTE  A structure of a radio frame according to FDD is shown.

The radio frame shown in FIG. 2 is a 3rd Generation Partnership Project (3GPP) TS 36.211 V8.2.0 (2008-03) "Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation (Release) 8) ".

Referring to FIG. 2, a radio frame consists of 10 subframes, and one subframe consists of two slots. Slots in a radio frame are numbered from 0 to 19 slots. The time taken for one subframe to be transmitted is called a transmission time interval (TTI). TTI may be referred to as a scheduling unit for data transmission. For example, one radio frame may have a length of 10 ms, one subframe may have a length of 1 ms, and one slot may have a length of 0.5 ms.

The structure of the radio frame is merely an example, and the number of subframes included in the radio frame or the number of slots included in the subframe may be variously changed.

Meanwhile, one slot may include a plurality of OFDM symbols. How many OFDM symbols are included in one slot may vary depending on a cyclic prefix (CP).

3 is 3GPP In LTE  An example diagram illustrating a resource grid for one uplink or downlink slot.

Referring to FIG. 3, an uplink slot includes a plurality of orthogonal frequency division multiplexing (OFDM) symbols in a time domain, and includes NRB resource blocks (RBs) in a frequency domain. do. For example, in the LTE system, the number of resource blocks (RBs), that is, NRBs, may be any one of 6 to 110.

Here, an example of one resource block includes 7 × 12 resource elements including 7 OFDM symbols in the time domain and 12 subcarriers in the frequency domain, but the number of subcarriers and the number of OFDM symbols in the resource block is equal to this. It is not limited. The number of OFDM symbols or the number of subcarriers included in the resource block may vary. That is, the number of OFDM symbols may be changed according to the length of the above-described CP. In particular, 3GPP LTE defines that 7 OFDM symbols are included in one slot in the case of a normal CP, and 6 OFDM symbols are included in one slot in the case of an extended CP.

The OFDM symbol is used to represent one symbol period, and may be referred to as an SC-FDMA symbol, an OFDMA symbol, or a symbol period according to a system. The RB includes a plurality of subcarriers in the frequency domain in resource allocation units. The number NUL of resource blocks included in an uplink slot depends on an uplink transmission bandwidth set in a cell. Each element on the resource grid is called a resource element.

Meanwhile, the number of subcarriers in one OFDM symbol may be selected and used among 128, 256, 512, 1024, 1536, and 2048.

In 3GPP LTE of FIG. 3, a resource grid for one uplink slot may be applied to a resource grid for a downlink slot.

4 shows a structure of a downlink subframe in 3GPP LTE.

It may be referred to section 4 of 3GPP TS 36.211 V10.4.0 (2011-12) "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 10)".

The radio frame includes 10 subframes indexed from 0 to 9. One subframe includes two consecutive slots. Thus, the radio frame includes 20 slots. The time it takes for one subframe to be transmitted is called a transmission time interval (TTI). For example, one subframe may have a length of 1 ms and one slot may have a length of 0.5 ms.

One slot may include a plurality of orthogonal frequency division multiplexing (OFDM) symbols in the time domain. OFDM symbol is only for representing one symbol period in the time domain, since 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in downlink (DL), multiple access scheme or name There is no limit on. For example, the OFDM symbol may be called another name such as a single carrier-frequency division multiple access (SC-FDMA) symbol, a symbol period, and the like.

In FIG. 4, it is illustrated that 7 OFDM symbols are included in one slot by assuming a normal CP. However, the number of OFDM symbols included in one slot may change according to the length of a cyclic prefix (CP). That is, as described above, according to 3GPP TS 36.211 V10.4.0, one slot includes 7 OFDM symbols in a normal CP, and one slot includes 6 OFDM symbols in an extended CP.

A resource block (RB) is a resource allocation unit and includes a plurality of subcarriers in one slot. For example, if one slot includes 7 OFDM symbols in the time domain and the resource block includes 12 subcarriers in the frequency domain, one resource block includes 7 × 12 resource elements (REs). It may include.

The DL (downlink) subframe is divided into a control region and a data region in the time domain. The control region includes up to three OFDM symbols preceding the first slot in the subframe, but the number of OFDM symbols included in the control region may be changed. A physical downlink control channel (PDCCH) and another control channel are allocated to the control region, and a PDSCH is allocated to the data region.

As disclosed in 3GPP TS 36.211 V10.4.0, a physical channel in 3GPP LTE is a physical downlink shared channel (PDSCH), a physical downlink shared channel (PUSCH), a physical downlink control channel (PDCCH), and a physical channel (PCFICH). It may be divided into a Control Format Indicator Channel (PHICH), a Physical Hybrid-ARQ Indicator Channel (PHICH), and a Physical Uplink Control Channel (PUCCH).

The PCFICH transmitted in the first OFDM symbol of the subframe carries a control format indicator (CFI) regarding the number of OFDM symbols (that is, the size of the control region) used for transmission of control channels in the subframe. The user device first receives the CFI on the PCFICH and then monitors the PDCCH.

Unlike the PDCCH, the PCFICH does not use blind decoding and is transmitted on a fixed PCFICH resource of a subframe.

The PHICH carries a positive-acknowledgement (ACK) / negative-acknowledgement (NACK) signal for a UL hybrid automatic repeat request (HARQ). An ACK / NACK signal for uplink (UL) data on the PUSCH transmitted by the user device is transmitted on the PHICH.

The Physical Broadcast Channel (PBCH) is transmitted in the preceding four OFDM symbols of the second slot of the first subframe of the radio frame. The PBCH carries system information necessary for the user equipment to communicate with the base station, and the system information transmitted through the PBCH is called a master information block (MIB). In comparison, system information transmitted on the PDSCH indicated by the PDCCH is called a system information block (SIB).

The PDCCH includes resource allocation and transmission format of downlink-shared channel (DL-SCH), resource allocation information of uplink shared channel (UL-SCH), paging information on PCH, system information on DL-SCH, and random access transmitted on PDSCH. Resource allocation of higher layer control messages, such as responses, sets of transmit power control commands for individual user devices in any group of user devices, activation of voice over internet protocol (VoIP), and the like. A plurality of PDCCHs may be transmitted in the control region, and the user device may monitor the plurality of PDCCHs. The PDCCH is transmitted on an aggregation of one or several consecutive control channel elements (CCEs). CCE is a logical allocation unit used to provide a PDCCH with a coding rate according to a state of a radio channel. The CCE corresponds to a plurality of resource element groups. The format of the PDCCH and the number of bits of the PDCCH are determined according to the correlation between the number of CCEs and the coding rate provided by the CCEs.

Control information transmitted through the PDCCH is called downlink control information (DCI). DCI can be used for resource allocation of PDSCH (also called DL grant), PUSCH resource allocation (also called UL uplink grant), and transmit power control commands for individual user devices in any group of user devices. And / or activation of Voice over Internet Protocol (VoIP).

The base station determines the PDCCH format according to the DCI to be sent to the user equipment, and attaches a cyclic redundancy check (CRC) to the control information. In the CRC, a unique radio network temporary identifier (RNTI) is masked according to an owner or a purpose of the PDCCH. If the PDCCH is for a specific user device, a unique identifier of the user device, for example, a cell-RNTI (C-RNTI) may be masked to the CRC. Alternatively, if the PDCCH is for a paging message, a paging indication identifier, for example, p-RNTI (P-RNTI), may be masked to the CRC. If the PDCCH is for a system information block (SIB), a system information identifier and a system information-RNTI (SI-RNTI) may be masked to the CRC. A random access-RNTI (RA-RNTI) may be masked in the CRC to indicate a random access response that is a response to the transmission of the random access preamble of the user equipment.

In 3GPP LTE, blind decoding is used to detect the PDCCH. Blind decoding is a method of demasking a desired identifier in a cyclic redundancy check (CRC) of a received PDCCH (referred to as a candidate PDCCH) and checking a CRC error to determine whether the corresponding PDCCH is its control channel. . The base station determines the PDCCH format according to the DCI to be sent to the user equipment, attaches the CRC to the DCI, and masks a unique identifier (referred to as Radio Network Temporary Identifier (RNTI)) to the CRC according to the owner or purpose of the PDCCH. do.

According to 3GPP TS 36.211 V10.4.0, the uplink channel includes a PUSCH, a PUCCH, a sounding reference signal (SRS), and a physical random access channel (PRACH).

5 shows a structure of an uplink subframe in 3GPP LTE.

Referring to FIG. 5, an uplink subframe may be divided into a control region and a data region in the frequency domain. A physical uplink control channel (PUCCH) for transmitting uplink control information is allocated to the control region. The data area is allocated a PUSCH (Physical Uplink Shared Channel) for transmitting data (in some cases, control information may also be transmitted).

PUCCH for one user equipment is allocated as an RB pair in a subframe. Resource blocks belonging to a resource block pair occupy different subcarriers in each of a first slot and a second slot. The frequency occupied by RBs belonging to the RB pair allocated to the PUCCH is changed based on a slot boundary. This is called that the RB pair allocated to the PUCCH is frequency-hopped at the slot boundary.

The user equipment may obtain frequency diversity gain by transmitting uplink control information through different subcarriers over time. m is a location index indicating a logical frequency domain location of a resource block pair allocated to a PUCCH in a subframe.

The uplink control information transmitted on the PUCCH includes a hybrid automatic repeat request (HARQ) acknowledgment (ACK) / non-acknowledgement (NACK), a channel quality indicator (CQI) indicating a downlink channel state, and an uplink radio resource allocation request. (scheduling request).

The PUSCH is mapped to the UL-SCH, which is a transport channel. The uplink data transmitted on the PUSCH may be a transport block which is a data block for the UL-SCH transmitted during the TTI. The transport block may be user information. Alternatively, the uplink data may be multiplexed data. The multiplexed data may be a multiplexed transport block and control information for the UL-SCH. For example, control information multiplexed with data may include a CQI, a precoding matrix indicator (PMI), a HARQ, a rank indicator (RI), and the like. Alternatively, the uplink data may consist of control information only.

<Reference Signal>

Hereinafter, the reference signal will be described.

Reference signals are generally transmitted in sequence. As the reference signal sequence, any sequence may be used without particular limitation. The reference signal sequence may use a PSK-based computer generated sequence. Examples of PSKs include binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK). Alternatively, the reference signal sequence may use a constant amplitude zero auto-correlation (CAZAC) sequence. Examples of CAZAC sequences are ZC-based sequences, ZC sequences with cyclic extensions, ZC sequences with truncation, etc. There is this. Alternatively, the reference signal sequence may use a pseudo-random (PN) sequence. Examples of PN sequences include m-sequences, computer generated sequences, Gold sequences, and Kasami sequences. In addition, the reference signal sequence may use a cyclically shifted sequence.

The reference signal may be classified into a cell-specific RS (CRS), an MBSFN reference signal, and a UE-specific RS. The CRS is a reference signal transmitted to all terminals in a cell and used for channel estimation. The MBSFN reference signal may be transmitted in a subframe allocated for MBSFN transmission. The UE-specific reference signal may be referred to as a reference signal received by a specific terminal or a specific terminal group in a cell (dedicated reference signal (DRS)). In the DRS, a specific terminal or a specific terminal group is mainly used for data demodulation.

Hereinafter, the CRS will be described.

6 To  8 shows some examples of CRS structures when using more than one antenna.

Specifically, FIG. 7 illustrates a CRS structure when the base station uses one antenna, FIG. 8 illustrates a case where the base station uses two antennas, and FIG. 9 illustrates a case where the base station uses four antennas.

6 to 8 may refer to section 6.10.1 of 3GPP TS 36.211 V8.2.0 (2008-03). In addition, the CRS structure may be used to support the features of the LTE-A system. For example, it may be used to support features such as Coordinated Multi-Point (CoMP) transmission and reception scheme or spatial multiplexing. In addition, the CRS may be used for channel quality measurement, CP detection, time / frequency synchronization, and the like.

6 to 8, in the case of multi-antenna transmission in which a base station uses a plurality of antennas, there is one resource grid for each antenna. 'R0' represents a reference signal for the first antenna, 'R1' represents a reference signal for the second antenna, 'R2' represents a reference signal for the third antenna, and 'R3' represents a reference signal for the fourth antenna. Positions in subframes of R0 to R3 do not overlap with each other. ℓ is the position of the OFDM symbol in the slot ℓ in the normal CP has a value between 0 and 6. In one OFDM symbol, a reference signal for each antenna is located at six subcarrier intervals. The number of R0 and the number of R1 in the subframe is the same, the number of R2 and the number of R3 is the same. The number of R2 and R3 in the subframe is less than the number of R0 and R1. Resource elements used for the reference signal of one antenna are not used for the reference signal of another antenna. This is to avoid interference between antennas.

The CRS is always transmitted by the number of antennas regardless of the number of streams. The CRS has an independent reference signal for each antenna. The location of the frequency domain and the location of the time domain in the subframe of the CRS are determined regardless of the UE. The CRS sequence multiplied by the CRS is also generated regardless of the terminal. Therefore, all terminals in the cell can receive the CRS. However, the position and the CRS sequence in the subframe of the CRS may be determined according to the cell ID. The location in the time domain in the subframe of the CRS may be determined according to the number of antennas and the number of OFDM symbols in a resource block. The location of the frequency domain in the subframe of the CRS may be determined according to the number of the antenna, the cell ID, the OFDM symbol index l, the slot number in the radio frame, and the like.

The CRS sequence may be applied in units of OFDM symbols in one subframe. The CRS sequence may vary according to a cell ID, a slot number in one radio frame, an OFDM symbol index in a slot, a type of CP, and the like. The number of reference signal subcarriers for each antenna on one OFDM symbol is two. When a subframe includes N _RB resource blocks in a frequency domain, the number of reference signal subcarriers for each antenna on one OFDM symbol is 2 × N _RB . Therefore, the length of the CRS sequence is 2 × N _RB .

Equation 2 shows an example of the CRS sequence r (m).

Here, m is ^{_{0,1, ..., 2N RB max -1}} . 2N _RB ^max is the number of resource blocks corresponding to the maximum bandwidth. For example, 2N _RB ^max is 110 in 3GPP LTE system. c (i) is a pseudo random sequence in a PN sequence and may be defined by a Gold sequence of length-31. Equation 3 shows an example of the gold sequence c (n).

Where Nc = 1600, x ₁ (i) is the first m-sequence and x ₂ (i) is the second m-sequence. For example, the first m-sequence or the second m-sequence may be initialized for each OFDM symbol according to a cell ID, a slot number in one radio frame, an OFDM symbol index in a slot, a type of CP, and the like.

In a system having a bandwidth smaller than 2N _RB ^max , only a portion of the 2 × N _RB length may be selected and used in a reference signal sequence generated with a 2 × 2N _RB ^max length.

The CRS may be used for estimation of channel state information (CSI) in the LTE-A system. A channel quality indicator (CQI), a precoding matrix indicator (PMI), a rank indicator (RI), and the like may be reported from the terminal when necessary through the estimation of the CSI.

Hereinafter, the DRS will be described.

9 and 10 show some examples of DRS structures.

9 shows an example of a DRS structure in a normal CP. In a normal CP, a subframe includes 14 OFDM symbols. 'R5' represents a reference signal of the antenna for transmitting the DRS. Reference subcarriers are positioned at four subcarrier intervals on one OFDM symbol including a reference symbol. 10 shows an example of a DRS structure in an extended CP. In the extended CP, a subframe includes 12 OFDM symbols. Reference signal subcarriers on one OFDM symbol are located at three subcarrier intervals. This may be referred to Section 6.10.3 of 3GPP TS 36.211 V8.2.0 (2008-03).

The location of the frequency domain and the time domain within the subframe of the DRS may be determined according to a resource block allocated for PDSCH transmission. The DRS sequence may be determined according to the terminal ID, and only a specific terminal corresponding to the terminal ID may receive the DRS.

The DRS sequence may also be obtained by Equations 1 and 2 above. However, m in Equation 1 is determined by N _RB ^PDSCH . N _RB ^PDSCH is the number of resource blocks corresponding to a bandwidth corresponding to PDSCH transmission. The length of the DRS sequence may vary depending on the N _RB ^PDSCH . That is, the length of the DRS sequence may vary according to the amount of data allocated to the terminal. The first m-sequence (x ₁ (i)) or the second m-sequence (x ₂ (i)) of Equation 1 is a cell ID, a position of a subframe in one radio frame, a terminal ID, etc. in every subframe Can be initialized accordingly.

The DRS sequence may be generated for each subframe and applied in units of OFDM symbols. In one subframe, the number of reference signal subcarriers per resource block is 12 and the number of resource blocks is N _RB ^PDSCH . The total number of reference signal subcarriers is 12 x N _RB ^PDSCH . Therefore, the length of the DRS sequence is 12 × N _RB ^PDSCH . When generating the DRS sequence using Equation 1, m is 0, 1, ..., 12N _RB ^PDSCH -1. DRS sequences are mapped to reference symbols in order. First, a DRS sequence is mapped to a reference symbol in ascending order of subcarrier indexes in one OFDM symbol and then to the next OFDM symbol.

In the LTE-A system, the DRS may be used as a demodulation reference signal (DMRS) for PDSCH demodulation. That is, the DMRS may be a concept in which the DRS of the LTE Rel-8 system used for beamforming is extended to a plurality of layers. PDSCH and DMRS may follow the same precoding operation. DMRS may be transmitted only in a resource block or a layer scheduled by a base station, and maintains orthogonality with each other.

11 shows an example of a DMRS structure.

11 shows a DMRS structure of an LTE-A system supporting four transmit antennas in a normal CP structure. CSI-RS can use the CRS of the LTE Rel-8 system as it is. DMRS is transmitted in the last two OFDM symbols of each slot, that is, the sixth, seventh, thirteenth and fourteenth OFDM symbols. The DMRSs are mapped to the 1st, 2nd, 6th, 7th, 11th, and 12th subcarriers in the OFDM symbol in which the DMRS is transmitted.

In addition, the CRS may be used simultaneously with the DRS. For example, it is assumed that control information is transmitted through 3 OFDM symbols (l = 0, 1, 2) of the first slot in the subframe. CRS may be used for an OFDM symbol having an OFDM symbol index of 0, 1, and 2 (l = 0, 1, 2), and DRS may be used for the remaining OFDM symbols except for three OFDM symbols. In this case, by multiplying a downlink reference signal for each cell and transmitting the predefined sequence, the receiver may reduce interference of a reference signal received from an adjacent cell, thereby improving performance of channel estimation. The predefined sequence may be any one of a PN sequence, an m-sequence, a Walsh hadamard sequence, a ZC sequence, a GCL sequence, a CAZAC sequence, and the like. The predefined sequence may be applied in units of OFDM symbols in one subframe, and another sequence may be applied according to a cell ID, a subframe number, an OFDM symbol position, a terminal ID, and the like.

Performance Estimation in Dual Mobility Environments

As described above, there is an increasing demand for high speed data communication in a dual mobility environment such as high speed railway. The performance estimation of the channel received by the UE in the dual mobility environment will be described below.

12 illustrates an example of a dual mobility environment.

Referring to FIG. 12, at least one Remote Radio Head (RRH) 30 is configured along a movement path (eg, a railway of a high speed rail) of the UE 10 to provide a service to the UE 10 in a dual mobility environment. Can be. One or more RRHs 30 provide a service to the UE 10 through a base band unit (BBU) 40. UE 01 and RRH 40 then use the SFN channel. Here, the RRH 30 is a device in which the radio frequency portion of the base station is separated. The BBU 40 is a device in which the baseband portion of the base station is separated.

The UE 10 located in the means of transportation (e.g. a high speed rail) is connected to two RRHs RRH ₁ and RRH ₂ , respectively, by radio link. In this case, the RRH ₂ present in the direction in which the UE 10 moves and the RRH ₁ present in the direction in which the UE 10 has already passed are only opposite directions with respect to the UE 10, and the relative speeds are the same. Accordingly, the UE 10 receives a channel including a Doppler frequency from RRH ₁ and RRH ₂ , respectively. That is, the UE 10 receives a channel including two Doppler frequencies having different signs from RRH ₁ and RRH ₂ .

Figure 13a To  10c shows some simulation results showing channel characteristics in two Doppler frequency situations.

FIG. 13A illustrates a Doppler shift for a signal respectively received from a plurality of RRHs 30 connected to a moving UE 10. 13B shows the distance between the moving UE 10 and each RRH 30. And, FIG. 13C shows the strength of the signal received at the UE 10 moving from each RRH 30.

Meanwhile, the current LTE or LTE-A standard lacks a definition for a dual mobility environment. In particular, the current LTE or LTE-A standard does not specifically define the operation of a wireless terminal for two Doppler frequencies. Specifically, a typical wireless terminal in a dual mobility environment performs frequency estimation by performing single frequency tracking. In addition, a channel estimator having a very high complexity is required for the wireless terminal to perform dual frequency tracking in a dual mobility environment.

14A and 14B Dual Mobility  Some simulation results show the performance of channel estimation in the environment.

14A shows the adaptive performance of a radio link in a dual mobility environment. And, FIG. 14B shows the performance of a fixed reference channel (FRC) for the adaptive performance of the radio link shown in FIG. 11A.

As shown in Figs. 14A and 14B, in the case of the existing wireless terminal performing a single frequency tracking, the performance of channel estimation is greatly degraded when the moving speed is increased. In the case of a high speed scenario enabled UE (HUE) performing dual frequency tracking, even if the moving speed is increased, the performance of the channel estimation does not significantly decrease, but the complexity of the channel estimator is excessively increased, thereby increasing the amount of computation of the channel estimation. And disadvantages such as increased power consumption.

<Disclosure of this specification>

In order to solve the problems as described above, embodiments according to the present specification proposes a method of performing dual frequency tracking to prevent performance degradation in the SFN channel and not increasing the complexity of channel estimation.

1. A Plan for Recognizing SFN Channel Environment

First, in the SFN channel environment (ie, dual mobility environment) considering the RRH 30, the base station 20 needs to inform the UE 10 that the current situation is the SFN channel environment. In this case, the UE 10 recognizing that it is an SFN channel environment will perform the methods proposed herein. In the following description, the base station 20 is described in the concept of including any one of the RRH 30 and the BBU 40, or both the RRH 30 and the BBU 40.

In order to make the UE 10 aware of the SFN channel environment, the base station 20 may transmit a signal previously promised to the UE 10. In the following description, a signal for letting the UE 10 know that the SFN channel environment is referred to as DualFreqTrack.

At this time, the DualFreqTrack signal may be one bit of information. For example, when the value of DualFreqTrack is 1, it may indicate that it is an SFN channel environment. When the value of DualFreqTrack is 0, it may indicate that it is not an SFN channel environment.

The base station 20 may transmit DualFreqTrack through the following method.

1) Transmission via SIB (System Information Block)

The base station 20 may transmit DualFreqTrack using the SIB transmitted through the physical downlink shared channel (PDSCH). In this case, the UE 10 may not perform dual frequency tracking until the SIB is received through the PDSCH.

2) Transmission through MIB (Master Information Block)

The base station 20 may transmit DualFreqTrack using the MIB transmitted through the physical broadcast channel (PBCH). The MIB transmitted through the PBCH consists of a total of 40 bits. However, practically, the bits used to transmit MIB information are 30 bits, and the remaining 10 bits are configured as spares. DualFreqTrack can be transmitted by utilizing one of these 10 bits. In this case, the UE 10 may perform dual frequency tracking as soon as only the PBCH is received. In addition, the existing wireless terminal is designed to ignore the pre-configured 10-bit, it is backward compatible in terms of signaling. An example of the structure of the MIB including the DualFreqTrack may be as follows.

2. Measures to reduce the complexity of dual frequency tracking

In this specification, in order to reduce the complexity of dual frequency tracking in an SFN channel environment (that is, a dual mobility environment), it is proposed to perform channel estimation by dividing a reference signal into two groups.

15 conceptually illustrates a channel estimation method according to the present specification.

More specifically, the base station 20 may transmit the CRS as follows to reduce the complexity of the dual frequency tracking.

1) The BBU 40 divides the CRS to be transmitted by the number of antennas into an odd group and an even group. For example, when four antennas are used, the BBU 40 may divide a CRS to be transmitted into a group consisting of CRS 0 and CRS 2 and a group consisting of CRS 1 and CRS 3. Here, CRS 0 is a reference signal for the first antenna port, CRS 1 is a reference signal for the second antenna port, CRS 2 is a reference signal for the third antenna port, and CRS 3 is a reference signal for the fourth antenna port. .

2) The BBU 40 assigns the divided odd and even groups to different adjacent RRHs 30, respectively. For example, the BBU 40 may assign an odd group to RRH ₁ and an even group to RRH ₂ .

3) Data to be transmitted based on the CRS is transmitted through the RRH 30 to which the corresponding antenna port is allocated after each precoding is completed.

The UE 10 performs channel estimation as follows.

1) When DualFreqTrack is received, the UE 10 divides the received CRS into an odd group and an even group. For example, when using four antennas, the UE 10 divides the received CRS into a group consisting of CRT 0 and CRS 2 and a group consisting of CRS 1 and CRS 3.

2) The UE 10 estimates the dual doppler frequency for each group that is divided. At this time, the Doppler estimation for each group may be performed in the same manner as the conventional method. In addition, it can be determined that the statistical characteristics of the odd group and the even group are the same.

3) The UE 10 performs channel estimation using a channel estimation parameter preset for each group based on the double Doppler frequency estimated for each group. In this case, the channel estimation parameters may be different for each group.

4) The UE 10 performs automatic frequency control (AFC) based on a weighted-averaged value of the channel power weights of the groups for the dual frequency tracking result estimated for each group. .

16 is a conventional CRS channel Estimator  Shows an example of the structure . And  17 shows an example of a structure of a CRS channel estimator according to the present specification.

As described above, when channel estimation is performed by dividing a reference signal into two groups, dual frequency tracking is distributed to each group. That is, the CRS channel estimator according to the present specification can be implemented using only one additional Doppler frequency tracker and path masking. Exemplary structures of a conventional CRS channel estimator and a CRS channel estimator according to the present specification are as shown in FIGS. 16 and 17.

Therefore, according to the present specification, even in an environment in which two Doppler frequencies exist, channel estimation can be performed while minimizing an increase in the complexity of the channel estimator.

In addition, only the method of transmitting the DualFreqTrack signal among the methods disclosed herein is applied, and even when the method of dividing the reference signal into groups is not applied, blind estimation is performed after LS (Least Squares) channel estimation. The method proposed in this specification by dividing a plurality of channel groups may be applied.

So far, this specification has described CRS-based transmission. However, the methods proposed in this specification may also be applied to DMRS-based transmission. Specifically, it can be applied to DMRS-based transmission as follows.

1) RRH allocation by port

When performing single layer transmission to the RRH 30 divided into odd groups and even groups, port 7 and port 8 are allocated to one UE 10 and transmitted. In this case, substantially only single layer transmission is possible for the transmission for the resource element (RE). Since it is expected that there will be almost no situation in which multiple links exist in the SFN channel environment, per-port RRH allocation as described above will be possible.

2) RRH allocation per symbol

The DMRS signal is orthogonal covering applied to the 3rd, 6th, 9th, and 12th OFDM symbols for the transmission mode (TM) 7 based on the time axis, and 6, 7, 12 for the TM 8/9/10. By applying orthogonal covering to the 13th OFDM symbol, waste of resource elements is limited and the number of antenna ports is increased. For SequenceTM 8/9/10 for normal CP 6, orthogonal covering for OFDM symbols is shown in the following table.

Accordingly, in the case of TM7, the OFDM symbols 3 and 8 are grouped into one group, the OFDM symbols 6 and 12 are grouped into another group and assigned to the RRHs 30 of different groups, thereby describing the CRS. May be applied. In the case of TM8 / 9/10, if OFDM symbols 6 and 12 are grouped into one group, OFDM symbols 7 and 12 are grouped into one group and assigned to RRHs 30 of different groups to orthogonality. Transmission is possible without damage. In this case, compensation of channel estimation may be performed by compensating through each Doppler frequency estimated from CRS ports corresponding to the same group.

In addition, the operation of the base station for DMRS-based transmission may be performed in the same manner as the CRS-based transmission described above.

According to the methods described so far, by recognizing the SFN channel environment and limiting the port assignment of the base station, it is possible to prevent the performance degradation of the channel estimation in the dual mobility environment and at the same time minimize the increase in the complexity of the channel estimator of the UE 10 Can be.

18 is a flowchart illustrating a channel estimation method according to the present specification.

Referring to FIG. 18, the UE 10 receives a signal for DualFreqTrack from the base station 20 (S100). Here, DualFreqTrack is an indicator for notifying that communication is performed using the SFN channel. DualFreqTrack can be extracted from the MIB received via the PBCH. In particular, DualFreqTrack may be transmitted through one bit replaced for DualFreqTrack transmission of the reserved bits in the MIB.

The UE 10 receives two or more CRSs (S200). The UE 10 divides the received two or more CRSs into two groups based on the antenna port number (S300). In this case, the UE 10 may classify two or more CRSs into two groups only when a signal for DualFreqTrack is received. In detail, the UE 10 may divide two or more CRSs into an odd group and an even group. The UE 10 may be divided into an odd group of CRSs having an odd antenna port number, and an even group of CRSs having an even antenna port number. As such, CRSs divided into odd groups and even groups may be received through different RRHs 30.

The UE 10 performs Doppler frequency tracking on each of the divided groups (S400). The UE 10 estimates the SFN channel based on the Doppler frequency tracking result performed for each group (S500). Furthermore, the UE 10 may perform automatic frequency adjustment (AFC) based on a weighted average value for the SFN channel estimation result.

Embodiments of the invention may be implemented through various means. For example, embodiments of the present invention may be implemented by hardware, firmware, software, or a combination thereof.

In the case of a hardware implementation, the method according to embodiments of the present invention may include one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs). Field programmable gate arrays (FPGAs), processors, controllers, microcontrollers, microprocessors, and the like.

In the case of an implementation by firmware or software, the method according to the embodiments of the present invention may be implemented in the form of a module, a procedure, or a function that performs the functions or operations described above. The software code may be stored in a memory unit and driven by a processor. The memory unit may be located inside or outside the processor, and may exchange data with the processor by various known means. Specifically, this will be described with reference to FIG. 21.

19 illustrates a wireless communication system in which the disclosures herein are implemented. Block diagram .

The base station 20 includes a processor 21, a memory 22, and an RF unit 23. The memory 202 is connected to the processor 21 and stores various information for driving the processor 21. The RF unit 23 is connected to the processor 21 to transmit and / or receive a radio signal. The processor 21 implements the proposed functions, processes and / or methods. In the above-described embodiment, the operation of the base station may be implemented by the processor 21.

The user device 10 includes a processor 11, a memory 12, and an RF unit 13. The memory 12 is connected to the processor 11 and stores various information for driving the processor 11. The RF unit 13 is connected to the processor 11 and transmits and / or receives a radio signal. The processor 11 implements the proposed functions, processes and / or methods. In the above-described embodiment, the operation of the user device may be implemented by the processor 11.

The processor may include application-specific integrated circuits (ASICs), other chipsets, logic circuits, and / or data processing devices. The memory may include read-only memory (ROM), random access memory (RAM), flash memory, memory card, storage medium and / or other storage device. The RF unit may include a baseband circuit for processing a radio signal. When the embodiment is implemented in software, the above-described technique may be implemented as a module (process, function, etc.) for performing the above-described function. The module may be stored in memory and executed by a processor. The memory may be internal or external to the processor and may be coupled to the processor by various well known means.

In the exemplary system described above, the methods are described based on a flowchart as a series of steps or blocks, but the invention is not limited to the order of steps, and certain steps may occur in a different order or concurrently with other steps than those described above. Can be. In addition, those skilled in the art will appreciate that the steps shown in the flowcharts are not exclusive and that other steps may be included or one or more steps in the flowcharts may be deleted without affecting the scope of the present invention.

Claims (14)

A method for estimating a channel by a user equipment, Receiving at least two CRS (Cell-specific Reference Signal); Dividing the two or more CRSs into two groups based on an antenna port number; Performing doppler frequency tracking on each of the divided groups; And Estimating a single frequency network (SFN) channel based on a Doppler frequency tracking result performed for each group. The method of claim 1, wherein the step of dividing into two groups The at least two CRSs are classified into an odd group and an even group, wherein the CRS having an odd antenna port number is an odd group, and the CRS having an even antenna port number is an even group. The method of claim 2, wherein the CRS divided into odd groups and even groups The channel estimation method, characterized in that each received via a different RRH (Remote Radio Head). The method of claim 1, wherein the step of dividing into two groups The at least two CRSs are divided into two groups only when an indicator for notifying that communication is performed using the SFN channel is divided into two groups. The method of claim 4, wherein the indicator And extracting from a MIB (Master Information Block) received through a physical broadcast channel (PBCH). The method of claim 5, wherein the indicator And is transmitted on one of the spare bits in the MIB replaced to transmit the indicator. The method of claim 5, And performing automatic frequency control based on a weighted-averaged value of the SFN channel estimation result. A user equipment for estimating a channel, RF (Radio Frequency) unit for transmitting and receiving a wireless signal; And A processor for controlling the RF unit, wherein the processor Controlling the RF unit to receive two or more Cell-specific Reference Signals (CRSs); Divide the two or more CRSs into two groups based on the antenna port number; Perform Doppler frequency tracking for each of the separated groups; And And estimating a single frequency network (SFN) channel based on a Doppler frequency tracking result performed for each group. The system of claim 8, wherein the processor is The at least two CRSs are classified into an odd group and an even group, wherein the CRS having an odd antenna port number is an odd group, and the CRS having an even antenna port number is an even group. 10. The method of claim 9, wherein the CRS divided into odd groups and even groups User equipment, characterized in that each received via a different RRH (Remote Radio Head). The system of claim 8, wherein the processor is Only when an indicator for notifying that communication is performed using the SFN channel is received, the user device is characterized in that the two or more CRSs are divided into two groups. The method of claim 11, wherein the indicator User equipment, characterized in that extracted from the Master Information Block (MIB) received through the Physical Broadcast Channel (PBCH). 13. The method of claim 12, wherein the indicator And is transmitted on one of the spare bits in the MIB replaced to transmit the indicator. The system of claim 8, wherein the processor is And based on a weighted-averaged value of the SFN channel estimation result, performing a procedure of performing automatic frequency control.

Images